Skeletal muscle health is a key determinant of functional capacity in adults. It allows for locomotion and activities of daily living, and as a metabolically active tissue, it contributes substantially to energy expenditure. The amount of skeletal muscle is determined by the balance between anabolism (protein synthesis) and catabolism (protein degradation). As such, it is worthwhile to investigate any nutritional strategies that can maintain or promote net addition of muscle tissue, and can prevent age-related declines in skeletal muscle mass.
The importance of skeletal muscle is magnified in conditions of weight loss. A multitude of animal and clinical studies have shown that isocaloric substitution of dietary protein for carbohydrate within the Dietary Reference Intakes (DRI) promotes favorable outcomes during weight loss, including increased retention of lean body mass and improved insulin sensitivity. These results are typically attributed to satiety effects or to diet induced thermogenesis, which fail to consider the unique ability of dietary protein to affect metabolic signaling. The amount of carbohydrate and protein in a meal has a profound effect on the post-prandial metabolic activity of liver, skeletal muscle, and adipose. A study was conducted to evaluate the post-prandial responses of these tissues to a range of diets varied in the dietary carbohydrate / protein ratio. Consumption of a diet with a high carbohydrate / protein ratio induced a metabolic shift characterized by post-prandial hyperinsulinemia, reduced insulin sensitivity in skeletal muscle at the expense of adipose, an increased capacity for de novo lipogenesis, and activation of the integrated stress response in liver, all of which were associated with a decline in body composition quality. These data highlight an important, underappreciated aspect of dietary protein to favorably impact whole-body metabolic activity and body composition.
Numerous research groups have demonstrated the acute effects of dietary protein and / or leucine on muscle protein synthesis (MPS). However, it was unclear if this acute response would translate into net accretion of muscle mass and changes in body composition when applied over time. Rats were fed 3 meals per day of isocaloric, isonitrogenous diets that varied in protein source with different leucine enrichments (WHEAT, 6.8% leucine; or WHEY, 10.9% leucine) for 11 weeks. At both 2 and 11 weeks, MPS was acutely stimulated by consumption of a standard 4 gram meal in WHEY but not WHEAT. Changes in MPS resulted in increased muscle mass, reduced fat mass, and greater diet-induced energy expenditure. At 11 weeks, gastrocnemius muscle weight was greater while body fat % was lower in WHEY-consuming animals. Interestingly, WHEAT-consuming animals had larger epididymal fat pad mass and greater total fat mass despite identical caloric input. Additionally, WHEAT preferentially partitioned dietary energy to fat while WHEY partitioned energy to lean tissue. These outcomes were associated with an induction of genes in skeletal muscle involved in mitochondrial biogenesis (e.g., PGC-1α, Tfam, cytochrome B) by WHEY. These data suggest that repeated consumption of leucine-rich meals stimulates MPS and presents the cell with dynamic and increased energy demands, provoking a response in the molecular machinery responsible for oxidation and energy production.
It has been well-established that the essential, branched-chain amino acid leucine has a unique ability to promote anabolism in skeletal muscle. Leucine stimulates mammalian target of rapamycin (mTOR), which subsequently activates a signaling cascade that increases muscle protein synthesis (MPS) through initiation factors such as p70 S6 kinase (p70S6K) and eukaryotic initiation factor 4E binding protein1 (4E-BP1). Our research group has shown that the leucine content of a meal predicts the acute response of MPS based on its ability to elevate the post-prandial plasma leucine concentration. Once stimulated, and yet despite continued elevations in plasma leucine and associated translation initiation factors (e.g., p70S6K and 4E-BP1), MPS returns to basal levels ~3 hours after a meal. However, administration of additional nutrients in the form of carbohydrate, leucine, or both ~2 hours after a meal was able to extend the elevation of MPS. This effect was associated with decreases in translation elongation activity as well as with increases in the AMP / ATP ratio and in the activity of 5’ adenosine monophosphate-activated kinase (AMPK), a key cellular energy sensor. These data suggest that leucine-stimulated increases in MPS result in an acute cellular energy deficit that in turn is likely responsible for the subsequent restoration of MPS activity to basal levels.
A study was designed to determine if AMPK directly responds to the MPS-induced deficit in cellular energy by inhibiting translation initiation, thusly inhibiting subsequent MPS activity. Rapamycin was administered to inhibit the leucine-induced stimulation of MPS, and separately compound C was administered to inhibit AMPK activity. Rapamycin appropriately inhibited mTOR signaling (Akt) and presumably MPS activity; however, compound C did not inhibit AMPK as anticipated. Further analysis of the experiment was abandoned due to failure of the key inhibitor compound C.
In total, this research demonstrates that consumption of a whole meal enriched in leucine acutely impacts cellular energy by activating protein synthesis in muscle, which compounded over time, promotes a repartitioning of energy and favorable body composition. These findings suggest that a substantial proportion of the observed benefits from increased protein consumption during meals is due to metabolic signaling effects and the associated energy costs, which are not appropriately captured in the traditional “diet as substrate” paradigm.